Field of the Invention
The present invention relates to a process for the catalytic reduction of nitrogen oxides contained in exhaust gases from combustions, which process comprises bringing said exhaust gases into contact with a Beta Zeolite exchanged with suitable amounts of cobalt salts, in the presence of a light hydrocarbon as the reducing agent.
The present invention relates also to a catalyst for such a process, which catalyst comprises a Beta Zeolite exchanged with cobalt salts, characterized by a Co:Zeolite Al molar ratio of&lt;0.5.
The emission of nitrogen oxides (NO and NO.sub.2), commonly defined as "NO.sub.x ", from traveling sources, such as automobiles, and from stationary sources, such as gas turbines and power stations, is among the major sources of atmospheric pollution.
The most effective techniques for removing NO.sub.x from the exhaust gases from combustions, can be classified into two categories:
(1) flame treatment, which limit NO.sub.x generation;
(2) post-combustion treatments.
Typical flame treatments are carried out, e.g., by varying the reaction stoichiometry, or decreasing the combustion temperature. Such treatments make it possible the decrease in NO.sub.x formation to be attained with only limited success.
Among post-combustion treatments, the catalytic processes result to be the most advantageous ones.
At present, nitrogen oxide removal is acoomplished at the industrial level through the so said "selective catalytic reduction" process (SCR process) carried out with ammonia, in the presence of a catalyst constituted by V.sub.2 O.sub.5 /TiO.sub.2, at a temperature comprised within the range of from 300.degree. to 400.degree. C. A conversion of nitrogen oxides of about 90% is described, which, however, decreases under operating conditions. This process also suffers from considerable drawbacks, such as e.g.:
(a) catalyst cost, and short life time of said catalyst, due to poisoning and sintering phenomena;
(b) presence of unreacted ammonia which, per se, is a pollutant;
(c) reaction between ammonia and sulfates, leading to NH.sub.4 HSO.sub.4 and (NH.sub.4).sub.2 SO.sub.4, which are corrosive for the facility components and poison the catalyst;
(d) difficulty of storage, handling and use of ammonia in the facility.
Owing to these problems, processes were studied which, on the one side, make it possible the direct catalytic decomposition to be performed of NO.sub.x, and, on the other side, aim at catalytically reducing NO.sub.x in the presence of different reducing agents from ammonia.
For both said process types, the most promising catalysts resulted to be those of zeolitic type, exchanged with transition metal salts.
As regards the direct catalytic decomposition processes, which preferably use ZSM-5 Zeolite exchanged with Cu.sup.2+ (Iwamoto, Yahiro, Shokubai, Catalyst, 1989, 31, 112), the main drawbacks are:
short catalyst life, due to the fact that the active sites of said catalysts are poisoned by oxygen, increasing amounts of which are formed as the decomposition reaction proceeds; PA1 sharp decrease in conversion rate values if an excess of oxygen is present, as it normally occurs under the operating conditions.
Owing to these problems, research mainly developed towards catalytic processes for NO.sub.x reduction with reducing agents which are different from ammonia.
As possible reducing agents free from environmental problems, light hydrocarbons were taken into consideration. Their use allows to operate also in the presence of oxygen. For example, Hamada et al. (Appl. Catal., 64 L1-L4, 1990) describe the reduction of NO.sub.x into gases containing oxygen which uses, as the catalyst, Zeolites in acidic form and alumina and, as the reducing agent, propane or propene. The best results described by the Authors consist in a conversion of 65%, obtainable with an H-mordenite at 400.degree. C.
Iwamoto et al. Shokubai 32, (6), 430, 1990! use copper-exchanged Zeolites and lower hydrocarbons as the reducing agents. The obtainable results depend on both hydrocarbon and oxygen concentrations. In the presence of large oxygen excesses, or at not high hydrocarbon concentrations, a considerable decrease is observed in conversion rates.
EP 499 087 and EP 499 286 disclose a method for purifying oxygen-rich exhaust gases containing NO.sub.x, CO and methane, in the presence of Zeolites of Y type, ferrierite, mordenite, ZSM-5 and ZSM-11 exchanged with transition metal salts. The highest obtainable NO.sub.x conversion is 50% and is attained when a cobalt-exchanged ZSM-5 Zeolite (Co-ZSM-5) is used, and the reaction Is carried out at 400.degree.-500.degree. C.
U.S. Pat. No. 5,149,512 discloses a catalytic process for destroying NO.sub.x contained in exhaust gases from combustion processes, which uses, Zeolite catalysts having a silicon:aluminum ratio of&gt;2.5, exchanged with cations selected from cobalt, nickel, iron, chrome, rhodium and manganese. Preferably Zeolites of MFI and MOR types are used; however, also a Beta Zeolite exchanged with cobalt is generally mentioned. This process consists of bringing the combustion gases, containing NO.sub.x and oxygen, into contact with said Zeolites, in the presence of methane as the only reducing gas.
Patent application JP 05220403 discloses a catalyst for the reduction of the nitrogen oxides contained in exhaust gases, comprising a Beta Zeolite with a molar ratio of SiO.sub.2 :Al.sub.2 O.sub.3 comprised within the range of from 10 to 100, exchanged with at least one metal selected from Cu, Co, Ni, Fe and Pt, in such an amount that the molar ratio of metal to aluminum in said Zeolite is comprised within the range of from 0.5 to 2.
In particular, for exemplifying purposes, only mentioned is the use of a Beta Zeolite having a molar ratio of SiO.sub.2 :Al.sub.2 O.sub.3 of 40, which Zeolite is exchanged with Cu, to such an extent that the molar ratio of copper to aluminum in the Zeolite is of 0.53. When the values of this ratio are lower than 0.5, the desired catalytic activity is no longer obtained. The process of nitrogen oxides reduction is carried out in the presence of propene as the reducing agent and at a hourly space velocity of 420,000 h.sup.-1.